A lens centering method using laser interferometry

Abstract

This invention relates to a lens centering method using laser interferometry, comprising a coherent light source, a beam splitter, a converging mirror, a reference mirror, a camera, a lens under test, and a turntable. The beam emitted by the coherent light source is collimated to form a collimated beam, and then split by the beam splitter into a reference arm beam and a measurement arm beam. The reference arm beam is reflected by a spherical reference mirror to form a curved beam, which returns to the beam splitter. The measurement arm beam is converged by the converging mirror to the vicinity of the vertex of the lens under test, and then reflected back to the converging mirror and beam splitter. This invention, by employing interferometry, obtains the eccentricity or tilt of the lens under test by analyzing the interferogram. The interferogram can cover the entire camera target surface, and all pixels of the camera can participate in the analysis and calculation, improving the utilization rate of camera pixels. Simultaneously, optical interferometry can achieve nanometer-level accuracy, breaking through the limitations of traditional sub-micron measurement accuracy based on camera spot position.

Description

Technical Field

[0001] This invention relates to the field of optical lens center deviation measurement, and more specifically, to a lens centering method using laser interferometry. Background Technology

[0002] Traditional optical lens eccentricity measuring instruments typically use incoherent light as the light source, employing a self-collimation method to acquire a spherical center spot image of the mirror surface. The eccentricity and tilt of the mirror are determined by judging the position of this spot image on the camera surface, and a high-precision air-bearing turntable is generally used to provide a relatively fixed reference axis. To improve measurement accuracy, it is necessary to improve the motion accuracy of the air-bearing turntable and refine the spot pattern. Using a crosshair spot is easier to achieve sub-pixel resolution than a dot spot, or further employing a grid or three-line spot pattern can further improve the camera's resolution. Because this traditional lens eccentricity measuring instrument requires a self-collimated optical path through the mirror's center, the probe's travel and the selected converging lens focal length must match the curvature of the mirror being measured. Therefore, the probe travel of the equipment generally needs to be large, and multiple converging lenses with different focal lengths need to be configured and replaced. Furthermore, although the spot pattern is continuously improved, the image spot acquired by the camera is only a small portion of the camera's target surface, resulting in low pixel utilization and thus limiting measurement resolution and accuracy. Summary of the Invention

[0003] The purpose of this invention is to provide a lens centering method using laser interference technology.

[0004] To address the aforementioned technical problems, this invention utilizes all the target surface pixels of the camera, improving the utilization rate of the camera target surface and enhancing detection accuracy. Furthermore, to reduce the need for a long probe travel and frequent changes to the converging lens focal length, this invention avoids aligning the converging lens focus with the center of the lens sphere (i.e., the confocal position) for measurement. Instead, the converging lens focus is aligned only near the vertex of the lens under test. In traditional autocollimation systems, when the converging lens focus is aligned with the vertex of the lens under test, the returned light will not carry information about the lens's eccentricity or tilt. Therefore, to measure the eccentricity and tilt of the lens under test through its vertex, the traditional optical path needs to be improved.

[0005] Furthermore, it includes a coherent light source, a beam splitter, a converging mirror, a reference mirror, a camera, the lens under test, and a turntable; among which:

[0006] The light beam emitted by the coherent light source is collimated to form a collimated beam, and then split into a reference arm beam and a measuring arm beam by a beam splitter.

[0007] The reference arm beam is reflected by the spherical reference mirror to form a curved beam, which then returns to the beam splitter.

[0008] The beam of the measuring arm is focused by the converging mirror to the vicinity of the vertex of the lens under test, and then reflected back to the converging mirror and the beam splitter by the lens under test;

[0009] The camera receives the reference arm beam and the measurement arm beam, which are combined by a beam splitter, and forms interference fringes.

[0010] By analyzing the morphology and density changes of the interference fringes, the eccentricity or tilt of the lens under test relative to the rotation axis of the turntable can be determined.

[0011] Furthermore, the lens under test is in a defocused state relative to the converging lens, and this defocused state causes the measuring arm beam to carry information about the eccentricity or tilt of the lens under test.

[0012] Furthermore, the interference pattern received by the camera gradually changes from ring-shaped fringes to straight fringes during the focusing process of adjusting the distance between the converging lens and the lens under test. The eccentricity or tilt of the lens under test is determined based on the density change of the straight fringes.

[0013] Furthermore, the reference mirror is a spherical mirror with adjustable curvature, which is used to achieve a balance between improving measurement resolution accuracy and expanding the measurement range.

[0014] Furthermore, the focal length of the converging mirror is selected based on the curvature of the reference mirror and the curvature of the lens under test to ensure that the formed interference fringes cover the camera target surface.

[0015] Furthermore, the lens under test is mounted on a turntable, which drives the lens to rotate. By analyzing the change in the density of the linear interference fringes acquired by the camera with the rotation angle of the turntable, the eccentricity of the lens under test relative to the turntable and the eccentricity of the beam convergence point relative to the turntable through the converging lens are distinguished. The former refers to the component of the linear interference fringe density that changes with the rotation of the turntable, while the latter refers to the component of the linear interference fringes that does not change with the rotation of the turntable.

[0016] Furthermore, it also includes using a phase shifter to perform phase shifting processing on the reference mirror, and obtaining the phase information of the interferogram through phase-shifting interferometry to improve the calculation accuracy of eccentricity and tilt.

[0017] Furthermore, the lens under test can be a planar lens or a spherical lens. When the curvature of the lens changes, the defocus distance is adjusted to ensure that the interference fringes change from ring fringes to straight fringes.

[0018] Furthermore, based on the paraxial object-image relationship formula, the formula for calculating the defocus distance δ when the mirror under test is a plane can be derived as follows:

[0019] ;

[0020] in, For the focal length of the converging lens, Let be the radius of curvature of the spherical reference mirror; the same applies when the surface to be measured is a sphere.

[0021] Furthermore, the tilt amount of the lens under test It can be derived from the formula for the magnification relationship between objects, and the formula is as follows:

[0022] ;

[0023] in, This represents the change in the density of the linear interference fringes received by the camera. For coherent light source wavelength, Let be the radius of curvature of the spherical reference mirror. Let be the focal length of the converging lens. If the lens under test is spherical with a radius of curvature of r, then the eccentricity D of the lens under test can be calculated using the following formula:

[0024]

[0025] The beneficial effects of this invention are:

[0026] This invention employs interferometry to determine the eccentricity or tilt of the lens under test by analyzing the interferogram. The interferogram can cover the entire camera target surface, allowing all camera pixels to participate in the analysis and calculation, thus improving the utilization rate of camera pixels. Furthermore, optical interferometry can achieve nanometer-level precision, breaking through the limitations of traditional sub-micrometer measurement accuracy based on camera spot position.

[0027] This invention only requires adjusting the focus of the probe to near the vertex of the lens to be measured, and the defocusing amount can be controlled in the range of tens of millimeters, which greatly reduces the demand on the probe travel. At the same time, it does not require the replacement of the converging lens, which reduces equipment costs, improves measurement efficiency, and is suitable for measurement needs with space constraints. Attached Figure Description

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] Figure 1 This is a schematic diagram of the measurement optical path.

[0030] Figure 2 This is a schematic diagram of the probe focusing process. The left image shows the stripes when the focus is not yet adjusted, and the right image shows the stripes when the focus is adjusted.

[0031] Figure 3 This is an example of how the stripes received by the camera change as the turntable rotates.

[0032] Figure 4 This is a schematic diagram showing the change in interference fringe density obtained from interference fringe analysis.

[0033] In the picture:

[0034] 1. Coherent light source; 2. Beam splitter; 3. Camera; 4. Reference mirror; 5. Converging mirror; 6. Specimen to be tested; 7. Turntable. Detailed Implementation

[0035] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, illustrating only the basic structure of the invention, and therefore only show the components relevant to the invention.

[0036] like Figure 1 As shown, this invention discloses a lens centering method using laser interferometry, relating to laser measurement technology. The method includes a coherent light source, a beam splitter, a converging mirror, a reference mirror, a camera, a lens under test, and a turntable. Specifically: the beam emitted by the coherent light source is collimated to form a collimated beam, and then split by the beam splitter into a reference arm beam and a measurement arm beam; the reference arm beam is reflected by the spherical reference mirror to form a curved beam, which returns to the beam splitter; the measurement arm beam is converged by the converging mirror to the vicinity of the vertex of the lens under test, and then reflected back to the converging mirror and beam splitter; the camera receives the combined reference arm beam and measurement arm beam from the beam splitter, forming interference fringes; by analyzing the morphology and density of the interference fringes, the eccentricity or tilt of the lens under test relative to the turntable axis is determined.

[0037] This invention employs an improved optical path structure of a Michelson interferometer and utilizes optical interferometry to measure the eccentricity of the lens under test. For example... Figure 1 As shown, the probe part of the optical path structure includes a coherent light source 1, a beam splitter 2, a camera 3, a reference mirror 4, and a converging mirror 5, while the rest includes the lens to be tested 6 and a turntable 7.

[0038] The coherent light source 1 is coherent light, and the light beam needs to be collimated by a collimating lens. The collimated beam is split by a beam splitter 2. One beam is incident on the reference mirror 4 and returns to the beam splitter 2, called the reference arm beam; the other beam is incident on the converging mirror 5, converges near the vertex of the lens under test 6, returns from the lens under test 6, is collimated by the converging mirror 5, and returns to the beam splitter 2, called the measurement arm beam. The reference arm beam and the measurement arm beam are simultaneously received by the target surface of the camera 3 and interfere. The lens under test 6 is placed on a turntable 7. As the turntable 7 rotates the lens under test 6, the eccentricity or tilt of the lens under test 6 relative to the axis of rotation of the turntable 7 can be obtained by analyzing the interference fringes received by the camera 3.

[0039] The lens under test is in a defocused state relative to the converging lens. This defocused state allows the beam of the measuring arm to carry information about the eccentricity or tilt of the lens under test. The reference mirror is a spherical mirror with adjustable curvature, which is used to achieve a balance between improving measurement resolution and expanding the measurement range. The focal length of the converging lens is selected according to the curvature of the reference mirror and the curvature of the lens under test to ensure that the interference fringes formed cover the camera target surface.

[0040] The aforementioned reference mirror 4 is a spherical mirror. Figure 1 In the diagram, 'a' is a collimated plane beam, and 'b' is a curved beam. Plane beam 'a' is reflected by reference mirror 4 and transformed into curved beam 'b'. When the beam from the measuring arm, after passing through converging mirror 5, converges to the vertex of the mirror surface of the lens under test 6, the interference pattern observed by camera 3 is as follows: Figure 2 The annular interferogram in the image is obtained by adjusting the axial relative position of the probe to the lens 6 under test, i.e., the focusing process. Figure 2 The ring-shaped stripes in the middle will gradually decrease and transform into Figure 2 The linear interference pattern is shown in the diagram. At this point, the beam convergence point of the measuring arm beam after passing through the converging mirror 5 will no longer coincide with the vertex of the mirror surface of the lens under test 6. This is called the defocus state, and the axial distance from the beam convergence point to the vertex of the lens under test 6 is called the defocus distance. It is this defocus state that causes the measuring arm beam to carry information about the eccentricity or tilt of the lens under test 6.

[0041] During the focusing process, the interference pattern received by the camera gradually changes from ring-shaped fringes to straight fringes. The eccentricity or tilt of the lens under test is determined based on the density change of the straight fringes. The lens under test is mounted on a turntable, which drives the lens under test to rotate. By analyzing the changes in the straight interference fringes acquired by the camera with the rotation angle, the eccentricity of the lens under test and the eccentricity of the turntable are distinguished.

[0042] When the lens under test 6 is eccentric or tilted relative to the turntable, the density of the linear interference fringes received by the camera 3 will change with the rotation of the turntable. When the lens under test 6 is not eccentric or tilted relative to the turntable, the density of the linear interference fringes will be a constant value. During the rotation of the lens under test with the turntable, the component of the linear interference fringes density that changes with angle is used to determine the eccentricity or tilt of the lens under test 6 relative to the turntable, while the component of the linear interference fringes density that remains constant with angle is used to determine the eccentricity of the beam's convergence point relative to the turntable after passing through the converging lens. Figure 3 This is an example of the fringe shape received by camera 3 at different rotation angles of turntable 7 when the lens under test 6 is eccentric relative to turntable 7, but the convergence point of the beam through the converging lens is not eccentric relative to turntable 7. When the eccentricity or tilt of the lens under test 6 relative to turntable 7 is adjusted to zero, the density of straight fringe will be zero.

[0043] The lens to be tested, 6, can be a planar lens or a spherical lens. When the curvature of the lens changes, the defocus distance is adjusted to ensure that the interference fringes change from ring fringes to straight fringes.

[0044] The surface of the lens 6 under test can be flat or spherical. When the curvature of the lens 6 changes, the axial distance between the probe and the lens 6, i.e., the defocus distance, will change. The goal of adjusting the defocus distance is still to reduce the number of annular fringes received by the camera 3, thereby transforming them into straight fringes.

[0045] The curvature of the aforementioned reference mirror 4 can be adjusted to meet the requirements of resolution and measurement range based on the eccentricity and tilt of the measurement. The smaller the curvature of the reference mirror 4, the greater the defocus distance required for the measuring arm beam. Eccentricity or tilt of the same lens 6 will result in a greater density of straight interference fringes received by the camera 3, improving measurement resolution but also reducing the measurable range.

[0046] The choice of focal length for the converging mirror 5 is similar to the choice of curvature for the reference mirror 4. A smaller focal length results in higher measurement resolution but a smaller measurement range.

[0047] Example 1: Figure 1 The coherent light source 1 is a single-mode fiber laser source, collimated into a collimated laser beam by a fiber collimator. The collimated beam is then split by a beam splitter 2 with a 1:1 splitting ratio. One collimated beam is incident on a concave reference mirror 4 with a curvature of 1000 mm, where it becomes a converging beam b and returns to beam splitter 2. The other beam is incident on a converging mirror 5 with a focal length of 150 mm. The point of convergence is approximately 32 mm axially from the plane test mirror 6, and is located above the plane test mirror 6. After returning from the test mirror 6, the converging beam becomes a diverging beam, with its virtual focal point located below the plane test mirror 6 and symmetrical to the point of convergence relative to the plane test mirror 6. The diverging beam is then re-converged by the converging mirror 5 and returns to beam splitter 2. Both beams are simultaneously received by the target surface of camera 3 and interfere. At this point, the convergence angles of the two beams should be the same, and the interference fringes received by camera 3 will be straight fringes. If ring fringes still exist in the interference fringes, they can be eliminated by finely adjusting the distance between the probe and the lens under test 6. To obtain accurate information about the interferogram, a phase shifter is used to shift the phase of the reference mirror 4. The phase shift method yields precise phase information of the interferogram, where the tilt amount in the phase information is the fringe density. The lens under test 6 is placed on the turntable 7, which rotates the lens under test 6. The tilt amount of the lens under test 6 relative to the rotation axis of the turntable 7 is obtained by analyzing the interference fringes received by the camera 3. Figure 4 This is a schematic diagram showing the change in interference fringe density obtained from interference fringe analysis. Figure 4A polar coordinate system was established using the turntable angle and the interference fringe density. The lines connecting the points formed by the interference fringe densities at all collected angles constitute a circle, and the length of the line connecting the center of this circle to the origin of the coordinate system represents the invariant component of the interference fringe density. The radius of the circle is the component of the variation in interference fringe density. Stripe density Angle of turntable The changing relationship can be expressed by the formula:

[0048] ;

[0049] All angles collected and the corresponding stripe density By fitting the above formula, the invariant component of the interference fringe density can be obtained. and the variation component of interference fringe density .

[0050] The defocus distance δ can be calculated using the following formula:

[0051] ;

[0052] in, For the focal length of the converging lens, Let be the radius of curvature of the spherical reference mirror.

[0053] tilt of the lens under test Calculated using the following formula:

[0054] ;

[0055] in, This represents the change in the density of the linear interference fringes received by the camera. For coherent light source wavelength, Let be the radius of curvature of the spherical reference mirror. This is the focal length of the converging lens.

[0056] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A lens centering method using laser interferometry, characterized in that: It includes a coherent light source, a beam splitter, a converging mirror, a reference mirror, a camera, the lens under test, and a turntable; among which: The light beam emitted by the coherent light source is collimated to form a collimated beam, and then split into a reference arm beam and a measuring arm beam by a beam splitter. The reference arm beam is reflected by the spherical reference mirror to form a curved beam, which then returns to the beam splitter. The beam of the measuring arm is focused by the converging mirror to the vicinity of the vertex of the lens under test, and then reflected back to the converging mirror and the beam splitter by the lens under test; The camera receives the reference arm beam and the measurement arm beam, which are combined by a beam splitter, and forms interference fringes. By analyzing the shape and density of the interference fringes, the eccentricity or tilt of the lens under test relative to the rotation axis of the turntable can be determined. The lens under test is in a defocused state relative to the converging lens, and this defocused state causes the measuring arm beam to carry information about the eccentricity or tilt of the lens under test.

2. The lens centering method using laser interferometry as described in claim 1, characterized in that, The interference pattern received by the camera gradually changes from ring-shaped fringes to straight fringes during focusing. The eccentricity or tilt of the lens under test is determined based on the density change of the straight fringes, where the density of the straight fringes is the number of fringes per unit length.

3. A lens centering method using laser interferometry as described in claim 2, characterized in that, The reference mirror is a spherical mirror with adjustable curvature, used to achieve a balance between improving measurement resolution accuracy and expanding the measurement range.

4. A lens centering method using laser interferometry as described in claim 3, characterized in that, The focal length of the converging mirror is selected based on the curvature of the reference mirror and the curvature of the lens under test to ensure that the resulting interference fringes cover the camera target surface.

5. A lens centering method using laser interferometry as described in claim 4, characterized in that, The lens under test is mounted on a turntable, which drives the lens to rotate. By analyzing the change in the density of linear interference fringes acquired by the camera with the rotation angle, the lens eccentricity and the turntable eccentricity can be distinguished.

6. A lens centering method using laser interferometry as described in claim 5, characterized in that, It also includes using a phase shifter to perform phase shifting on the reference mirror, and obtaining the phase information of the interferogram through phase-shifting interferometry to improve the calculation accuracy of eccentricity and tilt.

7. A lens centering method using laser interferometry as described in claim 6, characterized in that, The lens under test can be a planar lens or a spherical lens. When the curvature of the lens changes, the defocus distance is adjusted to ensure that the interference fringes change from ring fringes to straight fringes.

8. A lens centering method using laser interferometry as described in claim 7, characterized in that, When the lens under test is a plane, the defocus distance δ can be calculated using the following formula: ； in, For the focal length of the converging lens, Let be the radius of curvature of the spherical reference mirror.

9. A lens centering method using laser interferometry as described in claim 8, characterized in that, Tilt amount when the lens under test is a plane Calculated using the following formula: ； in, This represents the change in the density of the linear interference fringes received by the camera. For coherent light source wavelength, Let be the radius of curvature of the spherical reference mirror. This is the focal length of the converging lens.

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